Just like a colon (:) means something different from a semi-colon (;) so does an em dash, an en dash, and a regular dash all mean something different. They look different, too. Here are the basics.
A regular dash, or - symbol, is used between numbers, as in "Jenny Jenny 864-5789" in the popular song :). If you're formatting your book professionally, this should be the primary location of any standard dashes.
An en dash, or – symbol, is a specific kind of dash used in two specific situations. It is called an "en dash" because it's the width of the letter N. I.e. it's just a bit longer than a normal dash, and is meant to be a joiner symbol rather than a set-apart symbol.
Case 1: You use an en dash when joining ranges of dates. So for example if you say the 1990s ranged from 1990–1999, that is an en dash in that spot.
Case 2: You use an en dash when you have two adjectives which need to go together. So for example saying that something is cold and dark is fine without an en dash. The item can be cold, and it can be dark, both on their own. However, if you say that something is sunshine–yellow those two words go together. You can't just say something is "sunshine" and have it be meaningful. Sunshine is a necessary part of yellow. In cases where the two words are a single description, they need a dash between them. That is the en dash.
In Lord of the Rings by J. R. R. Tolkien, he uses an en dash here:
“I am old, Gandalf. I don't look it, but I am beginning to feel it in my heart of hearts. Well-preserved indeed! Why, I feel all thin, sort of stretched, if you know what I mean: like butter that has been scraped over too much bread. That can't be right. I need a change, or something.”
Just saying "well" has little meaning. The words "well" and "preserved" go together as one unit.
An em dash is longer than both an en dash and a regular dash. This is, as you might guess by now, the width of the letter M and looks like this — with a bit more length.
The tradition is that an em dash has no spaces before or after it, just like an en dash.
Em dashes can be seen in poetry to help add pauses to an already stanza-interrupted style, such as in Emily Dickinson:
All forests—stintless stars—
As much of noon, as I could take—
Between my finite eyes—
I adore poetry, and I adore Dickinson, so I appreciate this here.
Here's a line from Robert B. Parker in The Godwulf Manuscript. I adore the Spenser series :).
"It is a matter of the utmost delicacy, Mr. Spenser"—he was looking at himself in the glass again—"requiring restraint, sensitivity, circumspection, and a high degree of professionalism."
That being said, in regular literature I don't like the em dash "stuck to" the words. I like having it separate from the words to clarify that this is a pause dash and not a connective (en) dash. Many times if people are reading on small screens those types of nuances are lost on them.
So I am fond of the em dash — and at the same time I am in favor of spaces on either side of it. This usage of a space-on-either-side is also promoted by the New York Times and the AP Stylebook. It is a matter of clarity.
In Word, if you type -- (dash dash) in a row it should auto change it into an em dash for you.
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